The present invention relates to a friction vacuum pump for use in a system for regulating the pressure in a vacuum chamber comprising a multistage turbomolecular vacuum pump section whose stages each consist of a row of stator vanes and rotor vanes.
In the pressure regulating system described in U.S. Pat. No. 5,944,049, a friction vacuum pump of this type is employed. The pressure regulating facility described in said patent relies on the principle that the pressure in the vacuum chamber, i.e., on the high vacuum side of the friction pump is, within a specific pressure range, dependent on the pressure on the fore-vacuum side of the friction pump. Pressure regulation is effected in such a manner that in the instance of a pressure in the vacuum chamber deviating from the setpoint pressure, the fore-vacuum pressure for the friction vacuum pump is changed in such a manner that the pressure in the vacuum chamber is returned back to its setpoint pressure once again.
The known pressure regulating process is disadvantageous in that the pressure created by the friction vacuum pump is dependent on the type of gas, meaning that the pressure regulating process implemented thereby is also dependent on the type of gas. This shall be explained with reference to drawing FIGS. 1 and 2. Depicted in drawing FIG. 1 is a partial section through a friction vacuum pump 1, as being employed in the pressure regulating process detailed in U.S. Pat. No. 5,944,049. Pump 1 comprises a housing 2, in which a rotor 3 and a stator 4 are accommodated. For the purpose of producing a gas supply, a turbomolecular pump section 6 with rotor vanes 7 and stator vanes 8 as the active pumping elements is provided on the high vacuum side (inlet 5), and on the fore-vacuum side (discharge 9) a molecular pump section 11 designed as a Holweck pump is provided. The active pumping elements of a Holweck pump 11 consist of a single or multi-turn thread 12 arranged on the stator or the rotor side. In the design example depicted, the schematically depicted thread 12 is located at the side of the rotor 5. The active pumping elements (vanes 7, 8, thread 12) form the pump chamber 16 located between rotor 3 and stator 4, said pumping chamber extending from the first row of rotor vanes 13 to the end of thread 12 on the fore-vacuum side.
Friction pumps of the kind detailed and depicted in drawing FIG. 1 are penetrating the market more and more owing to their fore-vacuum tolerance. With the transition from the turbomolecular pump section 6 to the molecular pump section 11, the flow characteristics for the gasses being pumped also change. The initially molecular flow changes in to a viscous flow.
Turbomolecular pump section 6 is of a multistage design. Each stage comprises one each row of rotor vanes 13 and a row of stator vanes 14. The rotor 3 carries the row of rotor vanes 13. The stator 4 consists of rows of stator vanes 14 and spacing rings 15 arranged in alternating fashion over each other, said distance rings being centered by housing 2.
Rotor 3 is bell-shaped. Details as to the bearing and drive arrangement are known and not depicted in detail.
In the design example depicted in drawing FIG. 1, the cross section of the pumping area of pump 1 reduces from the high vacuum to the fore-vacuum side. This is attained in turbomolecular pump section 6 by reducing the radial length of the vanes 7, 8, and in the molecular pump section 11 by decreasing the height of the ridge of thread 12.
From the diagram of drawing FIG. 2, the regulating properties of the pump 1 in accordance with drawing FIG. 1 are apparent. Plotted on the abscissa is the fore-vacuum pressure, and on the ordinate the high vacuum pressure of the friction vacuum pump each according to a logarithmic scale. The solid curve applies to that instance where only nitrogen is present in the vacuum chamber. Within the fore-vacuum pressure range of approximately 2 to 4 mbar there is a strong dependence of the high vacuum pressure on the fore-vacuum pressure. The desired high vacuum pressure regulation is easily possible within this range.
The dashed line depicted in drawing FIG. 2 applies to that instance where only the heavier gas SF6 is present in the vacuum chamber. There is hardly any dependence of the high vacuum pressure on the fore-vacuum pressure. The friction vacuum pump 1 depicted in drawing FIG. 1 might in this instance not be employed as a pressure regulator.
It is the task of the present invention to design a friction vacuum pump of the aforementioned type so that it may also be employed in the presence of heavier gases for the purpose of regulating the pressure in a vacuum chamber.
The task is solved by the present invention through the characteristic features of the patent claims.
Surprisingly, it was found that through the measures in accordance with the present invention the dependence on the type of gas may be influenced. The measures have the effect that also in the presence of heavy gases in the vacuum chamber, there is a pressure dependence between high vacuum pressure and fore-vacuum pressure which may be utilized for pressure regulating purposes.